1. Field of the Invention
This invention relates to a process for the reduction of nitrogen oxide (NOx) emissions in the exhaust gases of diesel engines or turbines for stationary or mobile applications, and more particularly, to a process suitable for use in a Selective Catalytic Reduction (SCR) system.
The SCR system represents a known and widely spread technology for the removal of oxides of nitrogen in the exhaust gases from turbines, boilers, burners, power plants and other plants utilizing fossil fuels in the heavy industry. This system is based on the creation of a reducing atmosphere over a catalyst in the presence of the NOx compounds present in the exhaust gases.
The selection of a reducing agent depends on the local conditions prevailing in the different geographical areas where SCR systems are used. Conditions such as pricing, legislation and logistics play a role in the choice of the reducing agent. Reducing agents that are commonly used with diesel engines are neat or aqueous ammonia (NH3), solid urea (NH2CONH2) or urea dissolved in water.
Anhydrous ammonia is, however, extremely hazardous, toxic and volatile. On exposure to air, at a sufficiently high temperature and pressure, anhydrous ammonia can combine with air to form a combination that can be lethal. These properties therefore result in problems with the safety aspects where the storage, transportation and handling of large quantities of ammonia are concerned. Urea as a non-toxic alternative to ammonia does not present the same extensive safety problems, and it can be converted to ammonia at a latter stage.
In the case of ammonia or urea, the reducing agent reacts with NOx compounds such as nitrogen oxide (NO) and nitrogen dioxide (NO2), in the presence of a catalyst, and at normal exhaust gas temperatures of 250-450xc2x0 C., to liberate free nitrogen (N2) and water. The catalysts used are generally known as DENOX catalysts.
When the reducing agent is urea, it undergoes pyrolysis at approximately 275xc2x0 C. into gaseous ammonia and cyanuric acid (HNCO) according to the following reaction:
NH2CONH2xe2x86x92NH3+HNCO 
The HNCO then reacts with the water in the exhaust gas as follows:
HNCO+H2O xe2x86x92NH3+CO2 
The CO2 does not participate further in the DENOX reaction, whereas the ammonia molecules subsequently react with the nitrogen oxide, NO, on the surface of the catalyst according to the reaction:
2NO+2NH3+xc2xdO2+CO2xe2x86x922N2+3H2O+CO2 
Storage and feeding systems for the selected reducing agent which are used in the various plants, vary in design and construction, but in general, most SCR systems are applied to stationary plants with good infrastructure. Consequently, access to electricity, heat, and sheltered surroundings provide solutions for the safe storage and operation of stationary systems.
Contrary to the above-mentioned stationary plants are mobile applications such as heavy-duty (HD) truck vehicles operated either on the road, or off road vehicles and equipment. The application of DENOX catalysts for SCR systems to be used in the automotive industry requires that several criteria are fulfilled. These criteria include high resistance to extreme climatic conditions such as subzero temperatures lower than the crystallization point of urea, which is xe2x88x9211xc2x0 C.
Resistance to high temperature is also a requirement, since high temperature leads to the reducing agent exhibiting high vapour pressure, and this causes problems during refueling and venting of the vehicles. In addition, vehicle vibrations caused during driving also necessitate proper solutions in order to run the operations in a trouble-free manner.
Thus, there are current demands for high durability and stability when introducing these systems to the automotive industry.
2. Description of the Prior Art
There have been several attempts to reduce the emissions of NOx from diesel engines.
U.S. Pat. No. 6,063,350, which is incorporated herein by reference, discloses a method for SCR NOx emission reduction in an exhaust gas from a lean-burn engine using an aqueous solution of urea. In this system, temperature fluctuations and formation of solid deposits are avoided by monitoring the quality, temperature and level of the urea solution in a storage vessel, using a modular assembly of different sensors mounted inside the urea storage vessel. Based on the sensed parameters, sensor signals are generated compared to reference values, and the flow of urea solution is controlled in response to these signals. A heater can be used to maintain the temperature of the urea solution.
It also states that precipitation of solids is avoided by recirculating urea through a line between the storage vessel and the injector, which injects urea to the exhaust gases. The rate at which this recirculation takes place helps to maintain the temperature of the urea solution at a sufficiently low level, such that urea is not permitted sufficient time at elevated temperature to hydrolyze to the extent that solids are deposited.
U.S. Pat. No. 6,209,315 incorporated herein by reference, discloses a method and a device for controlled feeding of a reducing agent in an SCR process for reducing NOx in exhaust gas. The reducing agent is pumped from a storage container to a pressure accumulator inserted between the storage container and the metering valve to the SCR catalyst. The quantity of reducing agent metered may be evaluated from the displacement of a sprung (sprin-loaded) diaphragm in the pressure accumulator in association with a pressure sensor.
In all tank systems, various types of equipment can be used to avoid freezing of the urea solution. The freezing point of a 32.5 w/w % aqueous urea solution is xe2x88x9211xc2x0 C. Insulation and various heaters supplied by the battery or other energy sources are mentioned in the prior art. These types of protective equipment are only effective provided a constant power source is available. Loss of battery power in cold weather will cause freezing and crystallization of the urea at sufficiently low temperatures. In ordinary tank systems, damage is seen as a result of the expansion during freezing. The tank and the contents of the tank such as sensors, pumps and other equipment installed inside may then be permanently damaged.
Several problems associated with the SCR systems currently in use for NOx reductions are corrosion of the different components in the system, crystallization of urea which leads to deposits in the lines, maintaining constant valve settings and unacceptable evaporative emissions. Subzero temperatures also cause freezing followed by destruction of the tank liner pumps. No known system has so far been able to run safely during all the different operating modes.
It is therefore an object of the invention to provide a safe, reliable SCR system for reducing NOx emissions from diesel systems.
It is another object of the invention to eliminate the crystallization problems leading to destruction of equipment, associated with freezing of a reducing agent such as urea.
It is yet another object of the invention to provide simple modular membrane equipment to be used in the SCR system.
It is yet another specific object of the invention to provide an SCR system in which any type of liquid reducing agent can be used.
It is yet another specific object of the invention to provide a gas as a pneumatic driving force behind the transfer of the reducing agent.
It is yet another specific object of the invention to provide a liquid as a hydraulic driving force behind the transfer of the reducing agent.
These objects are achieved by the present invention, which provides an improved process and a simple modular apparatus for SCR NOx reduction.
The invention described herein concerns a process for reducing the content of nitrogen oxides (NOx) in the exhaust gases of diesel engines or turbines for stationary or mobile applications/vehicles in an SCR system by providing a stored source of liquid reducing agent and a hydraulic or pneumatic displacement fluid, and feeding the stored reducing agent to the exhaust gases, said process comprising
transferring the liquid reducing agent from the external storage tank to one or more membrane storage tanks, each equipped with an inner bellow consisting of a non-permeable flexible membrane, and a hydraulic or pneumatic displacement fluid located outside the inner bellow,
filling up the flexible inner bellow with liquid reducing agent and simultaneously exerting pressure on the displacement fluid in the membrane storage tank until the feeding pressure is attained,
increasing the pressure of the displacement fluid in the membrane storage tank by transferring more fluid into the volume present outside the flexible inner bellow, and thus forcing the liquid reducing agent to leave the flexible inner bellow,
transferring the liquid reducing agent from the flexible inner bellow to the exhaust gases via a dosing valve and a mixing device.
The invention concerns also an apparatus for reducing the content of nitrogen oxides (NOx) in the exhaust gases of diesel engines or turbines for stationary or mobile applications/vehicles in an SCR system, by providing a stored source of liquid reducing agent and feeding the stored reducing agent to the exhaust gases in a process according to claim 1, the apparatus comprising an external storage tank for storing liquid reducing agent,
one or more membrane storage tanks, each equipped with an inner bellow consisting of a non-permeable flexible membrane, being adapted to expand and contract with the aid of a hydraulic or pneumatic displacement fluid located outside the inner bellow,
a compressing device for the regulation of flow of displacement fluid to and from the membrane storage tank,
a dosing device for regulation of flow of reducing agent to the mixing device,
a mixing device for mixing reducing agent with air.
The process according to the invention utilizes a tank system with a modular membrane unit, which is more beneficial compared to tank systems which do not have such a unit. The use of a gas such as air as a pneumatic force or a liquid as a hydraulic force, eliminates loss of reducing agent due to evaporation during refilling. Non-membrane systems have an equilibrium determined saturated vapour in the air above the reducing agent.
If urea is used, urea vapour results in urea crystals when the vapour is dried. Deposition of solid urea crystals in the valves, inlets and outlets of the system lead to malfunction of the equipment. When the membrane storage tank is used, the separation of the reducing agent from the air present ensures that no vapour from the reducing agent is present in the air part of the system. Thus, no problems associated with vapour in the air are observed.
If urea or ammonia is used as a reducing agent, tanks, valves and tubing etc. made of inexpensive brass components cannot be used as they corrode in the presence of these compounds. More expensive types of steel have to be used in the areas subjected to increased pressure. It is, however, not necessary to use steel components in the storage system used in the process according to the invention due to its unique construction and functioning.
Another advantage of the system used in the process according to the invention is that the membrane tank system is not destroyed if it is accidentally cooled down below the freezing point of the urea solution. The reason for this is that the flexible membrane, containing the reducing agent solution, always has the ability to cope with the expansion of the reducing agent. This ability is obtained by using a flexible material, which can expand and contract. Suitable materials are different types of rubber, for instance EPDM (ethylene propylene diene monomer) rubber. Other types can also be used, provided they are flexible by nature.
The inner side of the storage tank shell can be coated with foam rubber. A suitable layer a few millimeters in thickness has shown to be sufficient to absorb the small expansion of the flexible membrane caused by the freezing or crystallisation of urea. No permanent damage of the tank system can thus be induced, and when the temperature exceeds the freezing point of the reducing agent, causing the reducing agent to melt, then the functioning of all equipment is normalised. To avoid freezing, precautions similar to those used for non-membrane systems may be used for example heating.